(1) Field of the Invention
The present invention relates to a method wherein phosphorus nitrides are impregnated into the exposed surface pores of wood in order to impart flame resistance. In particular the present invention relates to the impregnation of the wood pores with a crystalline phosphorus nitride having the formula P.sub.3 N.sub.5.
(2) Prior Art
The closest prior art is believed to be disclosed in U.S. Pat. No. 4,044,104 (1977) to Cremer et al. This patent describes the preparation of non-aggregating, particulate phosphorus nitrides of the formula PN.sub.x where x is a number between 0.9 and 1.7. In particular this reference describes crystalline P.sub.3 N.sub.5 (PN 1.67). The phosphorus nitrides are prepared by elevated temperature heating of phosphorus nitrides, formed by reacting ammonia and a phosphorus halide at relatively low temperatures, in stages in an inert gas to yield low or high density (crystalline) phosphorus nitrides. There are numerous other references describing the preparation of phosphorus nitrides, particularly P.sub.3 N.sub.5.
The Cremer et al patent describes the use of the low density products in "cellulose" and suggests that the compound is useful for imparting flame retardant properties to textile materials. It is believed that the reference to "cellulose" as used in this patent relates to cellulose derived polymer fibers where the phosphorus nitride is blended into a melt prior to spinning the fibers. There would be relatively substantial amounts of the phosphorus nitrides per unit volume or weight of the fibers. In any event, there is no indication in this patent that the phosphorus nitrides are impregnated into an exposed surface in relatively small amounts per unit volume or weight.
German Offenlegungschrift No. 2,311,180 (1974) (CA82:59676W) describes regenerated cellulose fibers containing ten percent (10%) by weight of P.sub.3 N.sub.5 which are flame retardant. In this instance the whole fiber contains a relatively large amount of the P.sub.3 N.sub.5. There is no suggestion of surface treatment of the cellulose fibers.
German Offenlegungschrift No. 2,440,074 (1976) (CA85:7219s) describes the use of five percent (5%) by weight of P.sub.3 N.sub.5 in regenerated cellulose fibers to provide flame resistance. Again this use requires a relatively high level of the phosphorus nitride.
It has not occurred to those skilled in the art that a very small amount of the phosphorus nitrides per unit volume or weight of an article could be used to provide flame resistance by impregnating only a surface or surfaces of a porous substrate in order to provide flame resistance. In particular, there has been no suggestion by the prior art that wood might be treated in this manner.